CN VII provides motor innervation to the muscles of facial expression (Figure 20-2B). The facial nerve exits the skull through the stylomastoid foramen and immediately gives off the posterior auricular nerve and other branches that supply the occipitalis, stylohyoid, and posterior digastricus muscles and the posterior auricular muscle. CN VII courses superficial to the external carotid artery and the retromandibular vein, enters the parotid gland, and divides into the following five terminal branches: temporal, zygomatic, buccal, mandibular, and cervical nerves, which in turn supply the muscles of facial expression. Other muscles of the face include muscles of mastication (temporalis, masseter, and the medial pterygoid and lateral pterygoid muscles), which are innervated by the motor division of CN V-3.
One of the most common problems involving CN VII occurs in the facial canal, just above the stylomastoid foramen. Here, an inflammatory disease of unknown etiology causes a condition known as Bell's palsy
, where all of the facial muscles on one side of the face are paralyzed. Bell's palsy is characterized by facial drooping on the affected side, typified by the inability to close the eye, a sagging lower eyelid, and tearing. In addition, the patient has difficulty smiling, and saliva may dribble from the corner of the mouth. If the inflammation spreads, the chorda tympani and nerve to the stapedius muscle may be involved.